1. Field of the Invention
The present invention relates to a supply pump for a common rail type (accumulation type) fuel injection system used in a diesel engine having a plurality of cylinders.
2. Description of the Related Art
There is a demand for high pressure fuel injection, and common rail fuel injection systems are developed in recent years. A general idea of a common rail fuel injection system will be described in reference to FIG. 2 of the accompanying drawings. A conventional common rail fuel injection system 1 includes a supply pump 2, a common rail 3 and unit injectors 4. The supply pump 2 feeds a pressurized fuel to the common rail 3. The pressurized fuel is accumulated in the common rail 3 and injected to cylinders of an engine from the respective unit injectors 4. Timing and amount of fuel injection from the unit injectors 4 are controlled by ECU (not shown).
Referring to FIG. 2A, the supply pump 2 is operatively connected to a crankshaft 78 of the engine 86 via a power transmission mechanism 84 so that it is driven by the engine 86. A typical power transmission mechanism is a chain-and-sprocket mechanism, a belt-and-pulley mechanism or a gear train mechanism.
The supply pump 2 also has a valve for adjusting a flow rate of pressurized fuel, and ECU controls this valve such that a discharge pressure of the supply pump 2 becomes a desired common rail pressure.
The common rail pressure drops each time a fuel is injected to the cylinders of the engine 86. In order to maintain the common rail pressure to a particular value or range, a fuel delivery timing of the supply pump 2 is synchronized with a fuel injection timing of the unit injectors 4 in the conventional common rail fuel injection system 1. The fuel delivery from the supply pump 2 takes place each time the fuel injection to the engine 86 takes place. Such a fuel injection system is disclosed in, for example, Japanese Patent Application, Kokai No. 4-308355.
However, the common rail fuel injection system 1 is different from a general fuel injection system in that the fuel delivery does not directly influence the fuel injection. Thus, the supply pump 2 does not necessarily feed the pressurized fuel to the common rail 3 each time the fuel is injected to the engine 86.
For example, if the engine has six cylinders, the fuel injection takes place six times while a crankshaft rotates twice. Accordingly, the general supply pump 2 feeds the fuel six times while the crankshaft rotates twice, with the fuel feed timing being in synchronization with the fuel injection timing. However, if it is possible to maintain the common rail pressure to a substantially constant value and insure an appropriate fuel injection, the supply pump 2 does not have to feed the fuel six times.
In consideration of the foregoing, a supply pump may be designed not to feed the fuel to the common rail in synchronization with the fuel injection timing. Specifically, the number of fuel delivery to the common rail 3 from the supply pump 2 during two rotations of the engine crankshaft 78 may differ from the number of the cylinders of the engine 86. For instance, a supply pump originally designed for a four-cylinder engine may be used in a six-cylinder engine. If this combination is feasible, a manufacturing cost will be reduced since the same supply pump is applicable to both of the four- and six-cylinder engines.
However, an excessively large load acts on the drive power transmission mechanism 84 between the supply pump 2 and the engine 86 unless the fuel delivery timing is optimum. In other words, if the timing of fuel supply from the supply pump is not appropriate, a chain tension and the like become so large, and therefore the same supply pump is not usable in different engines.